Conventionally, an image forming apparatus such as a copier performs post-processing such as stapling on a plurality pages of recording paper subjected to printing and thereafter successively discharges the recording paper to a discharge tray. For example, like a post-processing apparatus described in Japanese Laid-Open Patent Publication No. 2006-248685, in order to improve alignment of recording paper in a processing tray for performing post-processing (hereinafter, referred to as a post-processing tray), a front end side (a downstream side in a recording paper discharging direction) of the post-processing tray is generally inclined upward. Thereby, sheets of recording paper to be discharged and fallen to the post-processing tray slide the slope of the post-processing tray and run up against a rear end side of the post-processing tray so that their rear ends are aligned.
That is, in the conventional post-processing apparatus, the recording paper is discharged to the post-processing tray and slides down the slope of the post-processing tray, and then both sides of the sheets of recording paper are pressed with aligning plates to arrange them.
However, since the alignment in a discharging direction depends on free slide down of the recording paper, the alignment is unstable, and due to friction between sheets of recording paper and friction between a sheet of recording paper and a surface of the post-processing tray, the sheet of recording paper does not slide down the slope of the discharge tray and therefore sometimes stops in the middle of the slope. When a sheet of the recording paper does not slide down the slope of the post-processing tray to the end, the front end part of the sheet sticks out in the discharging direction of the recording paper, and this makes it difficult to align sheets of recording paper. In particular, in the case of a stapling mode, all sheets of recording paper can not be held together with staples and there is a risk of missing some of them.
As described above, when the post processing is carried out in a state where sheets of recording paper are not aligned in the post-processing tray, sheets of recording paper after the post processing are in the discharge state as shown in FIGS. 10A through 10C, and a user needs to align the sheets of recording paper again.
With respect to this matter, Japanese Laid-Open Patent Publication No. 8-282901 describes a technique to prevent sheets of recording paper from stopping in the middle of the slope of the bin by vibrating a bin group up and down when a first sheet of recording paper is discharged to a bin.
However, an object of the technique described in the Japanese Laid-Open Patent Publication No. 8-282901 is to improve irregularity in alignment caused by friction between the sheet of recording paper discharged to each bin first and a surface of each bin, but not to improve irregularity in alignment caused by friction between sheets of recording paper. Accordingly, since the second and subsequent sheets of recording paper can not smoothly slide down the slope of the discharge tray, the sheets may not aligned in the discharging direction.
Moreover, in the technique described in the Japanese Laid-Open Patent Publication No. 8-282901, since the bin is vibrated up and down by changing the direction of rotation of a bin lift motor instantaneously, the control is complicated, and further since the bin itself is vibrated up and down instantaneously, loads on the apparatus is also large.